Taxation is the forceful stealing of another person's property and wealth through the government and its agents.
It is completely immoral. It isn't moral for me to go to my neighbor and hold a gun to his head with the threat of either being shot or put in a cage of mine if he doesn't pay me half his earnings every year and there is nothing morally different about the government and its violent agents forcibly stealing someone's property either.
The implementation of government extortion programs such as income tax and the respective agencies that enforce them, such as the ATO, is just the mass scale purpetration of theft and the large scale enslavement of tens of millions of people.
What is the difference between a slave having their masters steal all the value of their labour from them at gun point, and the state stealing half the yearly earnings of the people it claims to have jurisdiction over? There is nothing fundamentally different here besides that in the former scenario, the slaves know they are slaves while in the latter, the slaves are under the false impression that they are free.
How could you, as ATO employees, possibly justify to yourselves assisting in the purpetration of this large scale forceful theft from the Australian people?